The invention is generally related to laser systems, and more particularly related to Optical Coherence Tomography systems and methods.
Optical Coherence Tomography (“OCT”) generates cross sectional imaging of tissues with micro-scale resolution by measuring the echo time delay of backscattered or backreflected light. Fourier Domain OCT (“FD-OCT”) can obtain a high sensitivity and imaging speed by acquiring the optical spectrum of light backscattered from a sample interfered with light from a single stationary reference reflector using an interferometer. Swept-Source OCT (“SS-OCT”), time-encodes the wavenumber (or optical frequency) by rapidly tuning a narrowband light source over a broad optical bandwidth.
The high speed tunable laser sources for SS-OCT exhibit a nonlinear or non-uniform wavenumber vs. time [k(t)] characteristic. As such, SS-OCT interferograms sampled uniformly in time [S(t), e.g., using an internal digitizer clock] must be remapped to S(k) before Fourier transforming into the path length (z) domain used to generate the OCT image. This software remapping operation is undesirable due to decreased phase sensitivity (i.e., amplitude and phase errors introduced in the remapping procedure), image artifacts, computational inefficiency, and the bandwidth needed to record a remapping array [k(t)] for each OCT interferogram [S(t)].
Additionally, when operating at fast frequency sweep speeds, the laser frequency varies nonlinearly rather than linearly with time in SS-OCT. Therefore, an accurate and reliable recalibration of the interference output to equidistant spacing in wavenumber (k) or optical frequency (v) is necessary. Also, nonlinearity may arise from the specifics of the tunable element(s) in the source, which might be the type of tunable filter (e.g., mechanical rotating mirror in combination with a grating, fabry perot, etc.), the operation of the tunable filter or time varying operation (drift) of the tunable filter. Analog-Digital (“A/D”) converter cards that operate at lower frequencies, i.e. low-speed digitizers, are not practical for the high-speed OCT systems. Moreover, most high speed A/D converter cards have a timing jitter of the generated clocking signal, which can degrade images recorded using SS-OCT. Most high-speed digitizers are very selective about the timing parameters of their external clock, i.e. duty cycle, pulse shape, amplitude, jitter, etc. Thus, the external clocking is difficult without some way of processing, whether analog or digital.
High-speed digitizers operating with an internal sample clock and swept-source lasers used for OCT must have a calibration performed between the non-linear sweep of the laser and the constant sampling frequency of the digitizer. This may be accomplished using software post-processing interpolation method of a linearly-sampled (in time) calibration k-space wavemeter signal or a non-uniform Fourier transform. Despite these approaches, obtaining a uniformly-sampled signal in wavenumber (k) or optical frequency (ν) improves OCT image quality and display rate. Unfortunately, today's high-speed digitizers have stringent requirements on the continuity of the amplitude and frequency of the external sample clock input, and disruptions in the wavemeter signal for example at times when the laser is between successive sweeps can disable the digitizer's internal sampling circuitry. The embodiments described herein solve these problems, as well as others.